It is well known to form a toothed disc by cold forming the disc from a steel blank and then forming the desired teeth thereon. Such discs can be used for a variety of purposes, such as flex plates for internal combustion engines, etc. In the specific case of flex plates, the teeth formed on the periphery of the disc are engaged by the engine's starter motor to rotate the crankshaft of the engine to start the engine.
For example, U.S. Pat. Nos. 5,152,061; 5,203,223; 5,237,744; and 5,404,640 to Himmeroeder, which are assigned to the assignee of the present invention and the contents of each of which patents are incorporated by reference herein, disclose methods of cold forming a toothed disc from a steel blank. In the first three Himmeroeder patents mentioned above, an annular thickened ring is cold formed about the periphery of the disc and a tooth forming device, in the form of a gear, engages this thickened ring and is synchronously rotated about the periphery of the disc to press-form the desired teeth into the thickened ring to finish the disc.
While the methods taught in the first three above-mentioned patents to Himmeroeder have been commercially successful, when the teeth are formed solely by cold press forming, it has proven difficult to achieve tight tolerances in the shape of the teeth. In U.S. Pat. No. 5,404,640, Himmeroeder discloses that to achieve tighter tolerances, the forming of the teeth can be completed with a hobbing operation which removes undesired material accumulated in the teeth during the cold press-forming operations.
Further, the need in the Himmeroeder methods to rotate the cold formed disc with a tooth forming device and, in some cases, to perform subsequently hobbing on the teeth requires a significant amount of time to complete the manufacture of the disc and incurs a corresponding manufacturing expense.
U.S. Pat. No. 5,878,493, also to Himmeroeder and assigned to the assignee of the present invention and the contents of which are also incorporated by reference herein, discloses a method of cold forming a disc with an annular thickened ring and then forming the desired teeth about the thickened ring by indexed rotation of the disc and a hobbing operation to completely form each tooth. While the disclosed method provides for the formation of the teeth with very tight tolerances, the manufacturing time required to form each tooth by hobbing is significant, again resulting in a relatively high manufacturing cost.